Try to fixup aarch64 cross build for hydra.

.github/workflows/ci-nix.yaml

@@ -46,10 +46,10 @@ jobs:
         name: cardano-scaling
         authToken: '${{ secrets.CACHIX_CARDANO_SCALING_AUTH_TOKEN }}'
 
-    - name: ❓Aarch64
+    - name: ❓ Aarch64
       if: ${{ matrix.package == 'aarch64' }}
       run: |
-        nix build .#legacyPackages.x86_64-linux.aarch64.hydra-node
+        nix build .#legacyPackages.x86_64-linux.aarch64.hydra-node.components.exes.hydra-node
 
     - name: ❓ Test (Pure)
       if: ${{ matrix.package == 'hydra-plutus'

flake.nix

@@ -1,7 +1,17 @@
 {
   inputs = {
     nixpkgs.follows = "haskellNix/nixpkgs";
-    haskellNix.url = "github:input-output-hk/haskell.nix";
+    haskellNix.url = "github:input-output-hk/haskell.nix?ref=angerman/fix-aarch64-musl";
+    hackageNix = {
+      url = "github:input-output-hk/hackage.nix";
+      flake = false;
+    };
+    haskellNix.inputs.hackage.follows = "hackageNix";
+    iserv-proxy = {
+      url = "github:stable-haskell/iserv-proxy?ref=iserv-syms";
+      flake = false;
+    };
+    haskellNix.inputs.iserv-proxy.follows = "iserv-proxy";
     iohk-nix.url = "github:input-output-hk/iohk-nix";
     flake-parts.url = "github:hercules-ci/flake-parts";
     lint-utils = {

nix/hydra/project.nix

@@ -54,5 +54,74 @@ pkgs.haskell-nix.project {
       # For this reason, we try to get away without re-installing lib:ghc for now.
       reinstallableLibGhc = false;
     }
+    {
+      # This is needed to stop GHC from trying to load libc++.a; I hope we'll
+      # have a better fix for this eventually. For now this will have to do.
+      packages.double-conversion.ghcOptions = [
+        # stop putting U __gxx_personality_v0 into the library!
+        "-optcxx-fno-rtti" "-optcxx-fno-exceptions"
+        # stop putting U __cxa_guard_release into the library!
+        "-optcxx-std=gnu++98" "-optcxx-fno-threadsafe-statics"
+      ];
+      # This is needed to stop failing when `git` can't be found. When
+      # cross compiling, git does not exist in the target context, and thus can
+      # not be executed.  Template Hsakell does not distinguish between native
+      # and target.
+      packages.gitrev.patches = [
+        (builtins.toFile "gitrev.patch" ''
+        diff --git a/src/Development/GitRev.hs b/src/Development/GitRev.hs
+        index b664692..603ad1b 100644
+        --- a/src/Development/GitRev.hs
+        +++ b/src/Development/GitRev.hs
+        @@ -62,7 +62,9 @@ runGit :: [String] -> String -> IndexUsed -> Q String
+         runGit args def useIdx = do
+           let oops :: SomeException -> IO (ExitCode, String, String)
+               oops _e = return (ExitFailure 1, def, "")
+        +      none :: SomeException -> IO (Maybe FilePath)
+        +      none _e = return Nothing
+        -  gitFound <- runIO $ isJust <$> findExecutable "git"
+        +  gitFound <- runIO $ isJust <$> findExecutable "git" `catch` none
+           if gitFound
+             then do
+               -- a lot of bookkeeping to record the right dependencies
+        '')
+      ];
+    }
+    # This is needed because plutus-tx, force loads the PlutusTx.Plugin, and then
+    # we fail with
+    #
+    #   > <no location info>: error:
+    #   >     Plugins require -fno-external-interpreter
+    #
+    # during the build.  Plugins and cross compilers are still WIP :-/
+
+    (pkgs.lib.mkIf (pkgs.hostPlatform.isMusl && pkgs.hostPlatform.isAarch64) {
+      packages.plutus-tx.patches = [
+        (builtins.toFile "plutus-tx.patch" ''
+        From 895a8a4af848ec29f9165fbff585f391d2c3358b Mon Sep 17 00:00:00 2001
+        From: Moritz Angermann <[email protected]>
+        Date: Sun, 10 Dec 2023 16:23:24 +0800
+        Subject: [PATCH] Update TH.hs
+
+        Just don't force load it.
+        ---
+        src/PlutusTx/TH.hs | 1 -
+        1 file changed, 1 deletion(-)
+
+        diff --git a/src/PlutusTx/TH.hs b/src/PlutusTx/TH.hs
+        index 49f26f6585e..02a0f927dd5 100644
+        --- a/src/PlutusTx/TH.hs
+        +++ b/src/PlutusTx/TH.hs
+        @@ -46,7 +46,6 @@ going to typecheck, and the result is always a 'CompiledCode', so that's also fi
+         -- | Compile a quoted Haskell expression into a corresponding Plutus Core program.
+         compileUntyped :: TH.Q TH.Exp -> TH.Q TH.Exp
+         compileUntyped e = do
+        -    TH.addCorePlugin "PlutusTx.Plugin"
+             loc <- TH.location
+             let locStr = TH.pprint loc
+             -- See note [Typed TH]
+        '')
+      ];
+    })
   ];
 }